In order to measure a propagation time of an optical signal between two optical devices connected by two distinct links, one for each transmission direction, it is known practice to measure an outgoing and incoming propagation time. An optical link consists of a succession of optical legs. An optical leg comprises notably an optical fiber. A measurement signal is sent by an optical device initiating the measurement to the adjacent optical device via the first optical link. The latter responds to the measurement signal to the initiating optical device via the second optical link. In an optical transmission network, when the respective lengths of the links of the two directions are not identical, the value of the propagation time estimated for each of the two links is contaminated with an error that is difficult to quantify.
In point of fact, many applications require a precise knowledge of this propagation time of an optical signal from one device to another. As an example, it is possible to cite methods of reserving resources or else methods of synchronizing optical devices.
In order to determine more precisely the propagation time of an optical signal on an optical link between two devices, there are methods, such as that described in patent FR 2896644, making it possible to reflect in the adjacent optical device a measurement signal transmitted by the device initiating the measurement over the optical link, optionally by means of other optical devices. The latter then receives a return signal on the same optical link and in the direction opposite to the propagation direction of the measurement signal. The initiating device can then determine the propagation time over the optical link. However, in certain cases, for example of long-distance links, amplification devices are provided between the two devices. An amplification device is used to amplify a signal received from the initiating device and sent to the adjacent device in the propagation direction. However, such an amplification device also plays a role of an isolator for the signals received in the direction opposite to the propagation direction on this optical link. Thus, when the method of patent FR 2896644 is implemented, the return signal is not transmitted beyond the last amplification device which is situated between the two devices, that is to say the first that the return signal encounters. In order to determine a propagation time on the optical link, it is then necessary to put in place notably measurements of a propagation time for each of the optical legs linking two amplification devices. Such a method is therefore complex to implement on an optical transmission network.